API Token's configured on project level notifications are lost after TeamCity is restart.

Repro steps:

1. Create a project level notification with an API Token
2. Restart TeamCity
3. View previously created notification, the API token is lost

Looking in the generated settings XML you can see that the API token is not stored as part of the XML. I believe the problem is in the SlackNotificationConfig class in the core project. When SlackNotificationConfig is instantiated with an Element it tries to read the "token" attribute, but the token attribute is never added in the getAsElement method.